Code Listing 5.1: Portage warning about blocked packages (with --pretend)
[blocks B ] mail-mta/ssmtp (is blocking mail-mta/postfix-2.2.2-r1)
Code Listing 5.2: Portage warning about blocked packages (without --pretend)
!!! Error: the mail-mta/postfix package conflicts with another package.
!!! both can't be installed on the same system together.
!!! Please use 'emerge --pretend' to determine blockers.
Ebuilds contain specific fields that inform Portage about its dependencies. There are two possible dependencies: build dependencies, declared in DEPEND and run-time dependencies, declared in RDEPEND. When one of these dependencies explicitly marks a package or virtual as being not compatible, it triggers a blockage.

While recent versions of Portage are smart enough to work around minor blockages without user intervention, occasionally you will need to fix it yourself, as explained below.

To fix a blockage, you can choose to not install the package or unmerge the conflicting package first. In the given example, you can opt not to install postfix or to remove ssmtp first.

You may also see blocking packages with specific atoms, such as <media-video/mplayer-1.0_rc1-r2. In this case, updating to a more recent version of the blocking package would remove the block.

It is also possible that two packages that are yet to be installed are blocking each other. In this rare case, you should find out why you need to install both. In most cases you can do with one of the packages alone. If not, please file a bug on Gentoo's bugtracking system.